Heating, ventilation, and/or air conditioning (HVAC) furnaces are widely used in commercial and residential environments for heating and otherwise conditioning interior spaces. To reduce emissions, HVAC furnaces may premix fuel/air completely prior to combustion. To help achieve this, HVAC furnaces sometimes comprise a premixer, such as a venturi premixer used to mix air and fuel prior to combustion. Some premixers may be designed for efficiently mixing fuel/air while also minimizing both pressure losses and the size of the premixer. In some furnaces, the air-fuel mixture outputted by the premixer may not be mixed to an effective level to provide for efficient burning of the air-fuel mixture.